1. Field of the Invention
This invention relates to a hydraulic-mechanical type continuously variable transmission for a vehicle, in which a hydraulic pump having an input rotary member coupled to a vehicle engine and a hydraulic motor having an output rotational axis coupled to a driving wheel are connected via a first oil passageway that has a low oil pressure during a normal load operation of the vehicle engine but has a high oil pressure during a reverse load operation of the vehicle engine and a second oil passageway that has a high oil pressure during the normal load operation but has a low oil pressure during the reverse load operation, thereby forming a closed hydraulic circuit. At least either the hydraulic pump or the hydraulic motor has a variable capacity.
2. Description of Background Art
A continuously variable transmission of the foregoing type is well-known and disclosed in Japanese Patent Publication Hei7-23746, for example.
In the foregoing continuously variable transmission, when engine braking is applied, the hydraulic motor functions as a pump while the hydraulic pump functions as a motor. In such a case, a first oil passageway tends to have an increased oil pressure. Specifically, the first oil passageway connecting an outlet side of the hydraulic pump to an inlet side of the hydraulic motor constitutes a closed hydraulic circuit together with a second oil passageway connecting an outlet side of the hydraulic motor and an inlet side of the hydraulic pump. In order to prevent this problem, throttle valves are provided between the first and second oil passageways in an existing continuously variable transmission, thereby protecting the first oil passageway against a large oil pressure when engine braking is applied.
However, such throttle valves cannot sufficiently reduce the oil pressure in the first oil passageway. Further, if a large transmission ratio (i.e. a low gear side) is produced by the continuously variable transmission immediately before engine braking is applied, the transmission ratio cannot be sufficiently reduced, which means that engine braking is sometimes too effective.